The present invention relates to apparatus for changing the pitch of a fan blade and, more particularly, is concerned with apparatus for varying the blade pitch of a ducted thrust fan having a plurality of closely spaced blades.
By-pass engines utilizing a ducted thrust fan driven by a turbine-type power plant have been recognized as exemplifying the current state-of-the-art in propulsion systems for aircraft. The fans have high by-pass ratios in the range of 5:1 or 6:1 and operate at fan pressure ratios of 1.4 to 1.5 at rated load. High blade solidity from closely spaced blades and high tip speeds are required for such ratios.
To optimize both take-off performance and cruise performance, it is desirable to vary the pitch of the fan blades. In addition, a variable pitch fan may also be used to produce reverse thrust during landings. Other reversing devices can be eliminated. Accordingly, a variable pitch thrust fan is currently favored by several propulsion system manufacturers.
Providing a pitch-change apparatus for a ducted thrust fan is complicated by several factors. The apparatus should be light-weight and compact since it will be mounted in the fan hub. The hub rotates at high speed to obtain the high by-pass and pressure ratios and, hence, centrifugal growth must be considered. The large number of blades in close proximity to one another at the hub and the large blade angles through which each blade must be turned to go from forward to reverse pitch place stringent requirements on the size of the apparatus and the magnitude of the displacements and forces produced by the apparatus. Non-linear variations in the blade twisting moments in the range from forward to reverse pitch through feathered pitch must be accommodated by the control loads carried by the apparatus. The possibility of fore-and-aft vibratory motions of the blade must also be accommodated in the pitch-change apparatus.
U.S. Pat. No. 3,902,822 by Andrews et al referenced above and having the same assignee as this application discloses a ducted thrust fan of the type to which the present invention relates. The fan has variable pitch blades which are located within a by-pass duct at the front end of a turbine power plant. The pitch-change mechanism utilizes bevel gearing which develops a constant mechanical advantage throughout the full pitch-range. Accordingly, the gearing and the mechanism operating the gearing must be designed to accept the peak input loads in a load curve proportional to the blade twisting moment. If the pitch-change apparatus had a mechanical advantage variable in substantially the same manner as the blade twisting moments, the input load curve would be more uniform without peaks over the full pitch-range and the pitch control apparatus could be made lighter.
It is, accordingly, a general object of the present invention to disclose a pitch-change apparatus having a relatively uniform input load curve to vary the pitch of a ducted thrust fan.